1. Field of the Invention
The invention relates to a fuel tank that stores a liquid fuel, and an fuel vapor treatment device equipped with the fuel tank.
2. Description of the Related Art
In a fuel tank that stores liquid fuel, fuel evaporates from a liquid surface of the fuel stored in the fuel tank, and fuel vapor is generated in the fuel tank. Japanese Patent Application Publication No. 2004-308483 (JP-A-2004-308483) describes a fuel vapor treatment device that introduces fuel vapor generated in a fuel tank into an intake passage of an internal combustion engine so that the fuel vapor is burned during the operation of the engine.
In this fuel vapor treatment device, fuel vapors generated in the fuel tank are introduced into a canister and temporarily adsorbed by an adsorbent provided in the canister. Then, during the operation of the engine, the air in the canister is drawn to the intake passage by the negative pressure in the intake passage, and purge is carried out to introduce the air into the canister from an air inlet passage. Due to the purge, the fuel adsorbed by the adsorbent is purged and introduced into the intake passage together with the air. The fuel purged from the adsorbent is then burned in the internal combustion engine.
That is, according to the above-described fuel vapor treatment device, fuel vapors generated in the fuel tank may be burned in the engine instead of being discharged to the atmosphere. In the meantime, in an fuel vapor treatment device mounted on a vehicle having an idling stop function of automatically stopping and restarting the operation of an internal combustion engine and hence designed to stop the internal combustion engine quite often, such as a hybrid vehicle or the like, the purge control is not executed very often. Thus, the amount of the fuel adsorbed by the adsorbent is unlikely to decrease, and the adsorbent is likely to be saturated. If the adsorbent is saturated, the adsorbent cannot adsorb any more fuel vapor. As a result, fuel vapor is discharged from the canister directly to the atmosphere through the air inlet passage.
Further, if the ambient temperature of outside air or fuel is high and fuel actively vaporizes in the fuel tank, even if a purge control is executed, newly generated fuel vapor is soon adsorbed by the adsorbent, and the adsorbent is therefore likely to become saturated. Thus, under a circumstance where fuel actively vaporizes, fuel vapor may be discharged directly to the atmosphere through the canister even if the fuel vapor treatment device is not mounted on a vehicle having the idling stop function.
Thus, in the fuel vapor treatment device described in Japanese Patent Application Publication No. 2004-308483 (JP-A-2004-308483), to restrain fuel vapor from being directly discharged to the atmosphere through the canister, a passage connecting the canister to the fuel tank is closed to seal the fuel tank when the engine is stopped, namely, during the cessation of purge.
If this configuration is adopted, fuel vapors are not introduced into the canister. Therefore, saturation of the adsorbent may be restrained, and the discharge of fuel vapor can be restrained from being discharged to the atmosphere through the canister.
However, fuel vapor cannot escape if the fuel tank is sealed, and therefore, the pressure in the fuel tank rises as more fuel is vaporized. Thus, if the fuel tank is sealed as described above, the fuel tank must have sufficient strength to withstand the rise in pressure. However, if the thickness of the fuel tank is increased to ensure sufficient strength, the weight and manufacturing cost of the fuel tank are increased.
Instead of the configuration in which the fuel tank is sealed as described above, it is also possible to adopt a configuration in which a canister equipped with a large-capacity adsorbent is provided to restrain the adsorbent from being saturated. However, if such a configuration is adopted, the size of the canister increases which may increase the size of the fuel vapor treatment device.
Thus, in order to effectively restrain fuel vapor from being discharged to the atmosphere from the canister without increasing the size of the canister, the weight of the fuel tank, and the like, it is desirable to suppress the generation itself of fuel vapor in the fuel tank.